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<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 0px;
      height: 0px;
      /* background-color: pink; */

      /* 方法一： */
      /* border-top: 100px solid skyblue;
      border-right: 100px solid yellow;
      border-bottom: 100px solid purple;
      border-left: 100px solid blue; */

      /* 方法二： */
      border: 100px solid transparent;
      border-bottom-color: blue;
    }
  </style>
</head>
<body>
  <div></div>
</body>
</html>